Clathrin assembly protein AP-3 is phosphorylated and glycosylated on the 50-kDa structural domain

AP-3 (AP180) in rat sympathetic neurons maintained in culture was analyzed by pulse-chase labeling with [35S]methionine to look for post-translational modifications. At early times, two lower molecular weight precursors of the mature species were detected. By 10 min, all of the AP-3 was found in the mature form which is stable for at least 9 h. We show here that at least one of these processing events is due to the addition of O-linked N-acetylglucosamine (GlcNAc) which is present on the mature form of the protein. Wheat germ agglutinin, a GlcNAc-specific probe, bound to AP-3 and the binding was blocked by excess GlcNAc but not by excess mannose. Purified AP-3, and AP-3 in coated vesicles derived from bovine brain, served as substrates for beta-D-galactosyltransferase which is specific for terminal GlcNAc residues. Analysis of the disaccharide released by beta-elimination indicated that single GlcNAc residues are attached to AP-3 through an O-glycosidic linkage to threonine or serine residues. In vivo 32P-labeled AP-3, the result of serine phosphorylation (Keen, J. H., and Black, M.M. (1986) J. Cell Biol. 102, 1325-1333), bound to wheat germ agglutinin-Sepharose indicating that phosphorylation and glycosylation can occur simultaneously on the same molecule. Both modifications have been mapped to the central 50-kDa structural domain that is responsible for the anomalous migration of AP-3. Consistent with localization to the nonclathrin binding domain, the O-GlcNAc modification does not play a discernible role in the interaction of AP-3 with clathrin.

Posttranslational processing of p21 ras proteins involves palmitylation of the C-terminal tetrapeptide containing cysteine-186

The p21 proteins of ras oncogenes are synthesized as precursors in the cytosol. After processing, which involves acylation, the products are associated with the plasma membrane in eucaryotic cells. The p21 overproduced in Escherichia coli, however, is not processed by acylation. A synthetic tetrapeptide of the p21 C terminus is used to identify the acylation site in eucaryotic p21 as cysteine-186. The same peptide of bacterial p21 is not acylated. Although p21 of Harvey murine sarcoma virus-transformed NRK cells can be metabolically labeled with either [3H]palmitate or [3H]myristate, the lipid moiety of the hydrophobic peptide is identified as palmitic acid. We suggest that the enzymatic mechanism for p21 palmitylation may be different from N-terminal myristylation of many other membrane proteins.